Indium Tin Oxide

ABSTRACT

A method of preparing indium tin oxide (ITO) and such an oxide per se are described. The method utilises a cryogenic process wherein an aqueous formulation of indium sulphate, ammonium sulphate and a tin compound, optionally in the presence of an organic polymer, are frozen to produce a solid; the solid is conditioned by heating it to cause crystallisation of water in the solid; the water is removed for example by freeze drying; and the solid is then calcined. The ITO produced may have a surface tin concentration of less than 2 and other desirable properties.

This invention relates to mixed metal oxides and particularly, althoughnot exclusively, relates to the preparation of indium tin oxide (ITO)and such an oxide per se.

Transparent conducting oxides (TCO's), of which ITO is one of the mostimportant, are essentially transparent to visible light but possessuseful electroconductivity. In general, TCO's, and, in particular, ITOcan be used to form transparent and/or conductive films, coatings,paints, and adhesives having one or more of a number or properties,including antistatic, anti rusting/corrosion, electricfield/electromagnetic wave shielding, UV shielding, anti reflection, lowreflective, anti streaking, improved scratch resistance, hardness,chemical resistance, and weather resistance. Among the applications inwhich the indium tin oxide is useful are printing electrode patterns,display devices, LC displays, touch screens, electroluminescent (EL)lamps, EMI shielding window, cathode ray tubes, architectural windows,flexible and rigid membrane switch displays, solar batteries, PDP(personal display devices), and glass security sensors.

The technology of choice for deposition of ITO (or indeed any TCO) in amanufacturing environment is d.c. magnetron sputtering using a metal orceramic target coupled with careful control of the atmosphere. Largequantities of ITO are utilised commercially to coat polyester sheet in avacuum roll process. This is often used as the top conducting electrodein EL-lamp displays. These typically have a sheet resistance of 50-500Ω/□ and transparency of 80-90%. There are, however, alternative methodsof depositing the clear conductive layer that offer much greater scopein the choice of substrate. This technology is based upon functionalinks utilising for example screen printing to build a device in layersonto practically any substrate. There is therefore a requirement for aTCO which can be incorporated as a pigment into a binder/solvent system,which can then be printed onto a substrate and dried down to give a filmwith the desired electrical and optical properties.

The requirements for a TCO are a large band gap >3 eV and a conductionband shape that ensures the plasma edge lies in the infrared. Typicallythe host structure will allow the introduction of a large number ofdegenerate carriers by a combination of non-stoichiometry andalieo-valent doping. Although a great many TCO's (both p-type andn-type) are known, ITO (an n-type) is believed to have the bestcombination of properties and is relatively easy to synthesise.

There are various known processes for the preparation of ITOcommercially. However, the ITO produced is relatively expensive.Furthermore, for some applications, ITO having different properties tothat currently available is desirable.

It is an object of the present invention to address the above-describedproblems.

According to a first aspect of the present invention, there is provideda process for preparation of ITO which includes the steps of:

-   -   (a) causing a liquid formulation which includes a solvent to        form a solid, wherein the formulation includes:        -   (i) an indium compound, a tin compound and ammonium            sulphate; or        -   (ii) (NH₄)In(SO₄)₂ and a tin compound;    -   (b) conditioning the solid;    -   (c) causing removal of solvent from the solid prepared in (b);    -   (d) calcining the solid after step (c) thereby to produce ITO.

The liquid formulation of step (a) is preferably an aqueous formulation.Thus said formulation preferably includes a major amount of water assolvent.

In the context of the present specification, a “major amount” means thatat least 70 wt %, suitably at least 80 wt %, preferably at least 90 wt%, especially at least 99 wt % of a specified material is present.

The solvent of step (a) preferably consists essentially of water.

The liquid formulation of step (a) is preferably at a temperature ofgreater than 0° C., more preferably greater than 5° C., especially atambient temperature prior to it being caused to form a said solid.

Step (a) preferably includes causing the liquid formulation to cool,suitably to a temperature which is at or below the freezing point of theliquid formulation. Suitably, the liquid formulation is introduced intoa low temperature environment which is at a temperature of less than−25° C., preferably less than −50° C., more preferably less than −100°C., especially less than −150° C. Said environment may be at ambientpressure.

Said environment may comprise a low boiling liquid at the temperaturestated. Said low boiling liquid is preferably inert and/or unreactivetowards any part of the formulation. Said liquid preferably comprise amaterial that is gaseous at STP. Said liquid preferably comprise liquidnitrogen, for example boiling liquid nitrogen.

In step (a), said liquid formulation is preferably caused to formparticles of solid. Suitably, less than 10 wt %, preferably less than 5wt %, more preferably less than 1 wt %, especially substantially noparticles formed in step (a) and treated in step (b) have a particlesize of less than 100 μm. Preferably, a major amount of said particlesare in the range 100 μm to 2 mm. Alternatively, it is possible toproduce particles in step (a) having a mean size of around 100 μmalthough such particle-size distributions are less preferred as thesmaller particle sizes present may result in loss of material/handlingdifficulties in the subsequent steps.

Said particles are preferably caused to form in step (a) by sprayingsaid liquid formulation into said low temperature environment. Said lowpressure environment, for example said low boiling liquid, may becontained within a receptacle closed at one end or may define a columnwherein the liquid formulation is atomised into a counter-current ofsaid low boiling liquid. The particles may then be isolated by anappropriate technique. For example, when said low temperatureenvironment is provided by a low boiling liquid, the particles may beseparated by liquid being decanted or the particles may be filtered toachieve separation.

The ratio of the number of moles of indium ions in said indium compoundto the number of moles of tin ions in said tin compound in said liquidformulation is suitably in the range 5 to 50, preferably 10 to 40, morepreferably 15 to 30, especially 18 to 23.

The ratio of the number of moles of ammonium ions in said ammoniumcompound (e.g. ammonium sulphate or ammonium indium sulphate) to thenumber of moles of tin ions in said tin compound in said formulation issuitably in the range 5 to 50, preferably 10 to 40, more preferably 15to 30, especially 18 to 23.

Preferably, the formulation comprises the materials of (a)(i). In thiscase, the ratio of the number of moles of indium ions in said indiumcompound to the number of moles of ammonium ions in said ammoniumsulphate is suitably in the range 0.6 to 1.5, preferably 0.8 to 1.3,especially 0.9 to 1.1. Also, in this case, the ratio of the number ofmoles of tin ions in said tin compound to the number of moles ofammonium ions in said ammonium sulphate is suitably in the range 0.01 to0.1, preferably 0.02 to 0.08, especially 0.03 to 0.07.

Said liquid formulation of step (a)(i) may include at least 0.4 wt % ofsaid tin compound (and preferably less than 5 wt %, more preferably lessthan 3 wt %), at least 2 wt % of ammonium sulphate (and preferably lessthan 10 wt %, more preferably less than 7 wt %), at least 10 wt % ofsaid indium compound (and preferably less than 30 wt %, more preferablyless than 25 wt %, especially less than 20 wt %), and at least 60 wt %of solvent (especially water) (and preferably less than 80 wt %, morepreferably less than 70 wt %).

Said liquid formulation of step (a)(ii) may include at least 0.4 wt % ofsaid tin compound (and preferably less than 5 wt %, more preferably lessthan 3 wt %), at least 12 wt % of (NH₄)In(SO₄)₂ (and preferably lessthan 35 wt %, more preferably less than 30 wt %, especially less than 25wt %) and at least 60 wt % of solvent (especially water) (and preferablyless than 80 wt %, more preferably less than 70 wt %).

Said tin compound used in steps (a)(i) or (a)(ii) is preferably a Sn(II)compound. It may be tin (II) sulphate or tin (II) fluoride. Preferably,it is tin (II) sulphate.

If said liquid formulation includes more than one type of indiumcompound or more than one type of tin compound, the above-mentionedamounts/ratios preferably refer to the sum of the amounts of indium andtin ions in such compounds as appropriate. Preferably, however, saidliquid formulation includes only a single type of indium compound and asingle type of tin compound.

Preferably, the indium compound and ammonium sulphate of step (a)(i) areadapted to produce (NH₄)In(SO₄)₂. Thus, the compounds of (a)(i) may uponcontact and/or reaction therebetween produce the compounds of (a)(ii).

The process of the first aspect may be carried out in the presence ofoxygen scavenging means that is suitably arranged to scavenge and/orreact with oxygen produced in the process, suitably to reduce the amountof oxygen that may be incorporated into the ITO prepared. As describedhereinafter, oxygen may fill oxygen vacancies in the ITO and,consequently, strip electrons from a conduction band of the ITO, therebyresulting in reduced conductivity.

Said scavenging means is preferably a chemical means, for example achemical compound which is able to react with decomposition productsproduced in the process, especially with such products produced in step(d). Such products may be produced by decomposition of compoundsincluded in said formulation of step (a). Said scavenging means, or adecomposition product thereof, suitably produced in step (d), may formcovalent bonds with products or intermediates produced by decompositionof compounds included in said formulation of step (a).

When compounds included in said formulation of step (a) include anindium sulphate compound (as is preferred), said scavenging means ispreferably arranged to scavenge a product of sulphate decomposition thatoccurs in step (d). If the process of step (d) is carried out in air,then said scavenging means may also react with oxygen in air.

Said scavenging means may be arranged to form SO₂ from decompositionproducts produced in step (d).

Said scavenging means may be arranged to produce CO₂ from decompositionproducts produced in step (d).

Said scavenging means may be arranged to produce H₂O from decompositionproducts produced in step (d).

Said scavenging means is suitably arranged to decompose in step (d).Thus, its decomposition temperature is preferably less than thetemperature at which calcination is carried out in step (d).

The amount of scavenging means in said formulation may be 0.9 to 2 timesthe amount required to fully reduce the products of the decomposition ofindium sulphate to indium oxide. The amount may be 1.1 to 1.8 times,preferably 1.3 to 1.7 times, more preferably 1.4 to 1.6 times,especially about 1.5 times the amount required as aforesaid.

The amount of scavenging means may be 0.9 to 2 times the moleequivalents of indium sulphate in said formulation. The amount may be1.1 to 1.8 times, preferably 1.3 to 1.7 times, more preferably 1.4 to1.6 times, especially about 1.5 times the amount described as aforesaid.

Said scavenging means is suitably a polymeric material and is preferablyan organic polymeric material. Said polymeric material preferablyincludes a repeat unit which consists of atoms selected only fromcarbon, hydrogen, oxygen and nitrogen atoms. Said repeat unit preferablyconsists of the aforementioned atoms.

Said polymeric material preferably has a molecular weight in the range5000 to 100000 amu, more preferably in the range 5000 to 50000 amu.

Said polymeric material preferably has a Tg of at least 25° C.,preferably at least 75° C., more preferably at least 125° C. The Tg maybe less than 400° C., preferably less than 200° C.

Said polymeric material is preferably water soluble. Preferably, atleast a 20 wt %, more preferably at least a 40 wt %, especially at leasta 50 wt % solution of said polymeric material in water at 25° C. can beprepared.

Said polymeric material is preferably completely water miscible at 25°C.

Said polymeric material is preferably wholly soluble in 0.25 molarindium sulphate solution at 25° C.

Said polymeric material may include a —NH₂ moiety in its repeat unit.Said polymeric material preferably includes an amide moiety in itsrepeat unit. Said polymeric material is preferably an acrylamide.

Said liquid formulation of step (a) of the method preferably includessaid scavenging means.

Said scavenging means is preferably dissolved in said liquid formulationselected for treatment in step (a) of the method. Each of the compoundsof step (a)(i) and (ii) selected for treatment in step (a) arepreferably in solution in said liquid formulation.

Said liquid formulation of step (a)(i) or (ii) may include at least 1 wt%, preferably at least 2 wt % (and preferably less than 5 wt %, morepreferably less than 4 wt %) of said polymeric material.

Said liquid formulation used in step (a) is preferably substantiallyhomogenous.

The solid prepared in step (a) suitably includes the compounds of (a)(i)or (ii), preferably (NH₄)In(SO₄)₂, optional scavenging means and frozensolvent, especially water, included initially in said liquid formulationof step (a).

In said conditioning step (b), a part of the solid is preferably causedto undergo a change, for example a physical change. Preferably, in step(b), the crystallinity of the solid is changed. Preferably, conditioningis arranged to increase the crystallinity of at least a component ofsaid solid. Preferably, conditioning is arranged to increase thecrystallinity of the solvent, especially water. Initially, the solvent,in admixture with the other components, may be in a relatively amorphousstate. Conditioning is preferably arranged to increase itscrystallinity. Preferably, said solvent is substantially crystallineafter said conditioning. Said conditioning may comprise devitrificationof said solid.

Step (b) may include raising the temperature of the solid (suitably byat least 5° C., preferably by at least 10° C.). Preferably, thedifference between the lowest temperature to which the solid issubjected in step (a) compared to the highest temperature to which it issubjected in step (b) is at least 50° C., more preferably at least 100°C. Preferably, conditioning includes raising the temperature of thesolid prepared in step (a); and maintaining the solid at a raisedtemperature for at least 5 minutes, preferably at least 15 minutes, morepreferably at least 25 minutes. Step (b) may include raising thetemperature in steps. It may be raised to a first raised temperature andheld at the temperature; and subsequently raised to a second temperatureand held at the temperature. Preferably in step (b), the maximumtemperature attained by the solid is less than 0° C., more preferablyless than −10° C., especially less than −20° C.

Step (b) preferably comprises annealing the solid.

Step (c) preferably includes causing vaporisation, preferablysublimation of the solvent. The step preferably includes applyingenergy, for example heat, to provide the latent heat of vaporisation ofthe solvent.

Step (c) is preferably carried out at less than ambient pressure. It maybe carried out at a pressure of less than 100 Pa, preferably at lessthan 50 Pa, more preferably at less than 20 Pa, suitably in a vacuum.Step (c) may be carried out at 10-20 Pa.

Step (c) is suitably carried out at a shelf temperature of greater than5° C., preferably greater than 15° C., more preferably greater than 20°C. Step (c) is preferably carried out wholly at a shelf temperature ofless than 80° C., more preferably less than 60° C.

Step (c) may involve raising the temperature of the solid of step (b),suitably gradually and in a vacuum; holding the solid at the raisedtemperature, suitably for at least one hour; raising the temperaturefurther and holding the solid at the raised temperature, suitably for atleast 1 hour, preferably at least 10 hours. Preferably, after step (c),the solid includes less than 1 wt %, more preferably substantially no,solvent, for example water

Step (d) preferably includes subjecting the solid to an environmentwherein the temperature is at least 400° C., preferably at least 600°C., more preferably at least 800° C. The temperature may be less than1200° C., preferably less than 1000° C. Suitably, the solid is subjectedto a temperature in the range 400° C. to 1200° C. (more preferably 800°C. to 1000° C.) for at least 10, preferably at least 20 minutes. It ispreferably held at a temperature within said ranges for less than 1hour.

Preferably, the solid is calcined in an inert gas atmosphere, forexample in a nitrogen atmosphere.

The ITO produced in the process preferably has a powder resistivity inthe range 0.1 to 0.5 Ω.cm, more preferably in the range 0.2 to 0.5 Ω.cmmeasured at less than 30% volume fraction, more preferably when measuredat less than 25% volume fraction. The BET surface area may be less than35m²/g, suitably less than 30m²/g, preferably less than 25m²/g, morepreferably less than 20m²/g, especially 17m²/g or less. The BET surfacearea may be at least 10 m²/g.

Advantageously, it has been observed by X-ray Photoelectron Spectroscopy(XPS) of the surface of the ITO powder prepared in the method that thetin and indium are more evenly distributed, leading to a relativelysmall increase in the surface tin concentration compared to thetheoretical surface tin concentration for ITO with tin and indiumdistributed entirely homogenously.

Thus, according to a second aspect of the invention, there is providedITO powder having a surface tin concentration in moles per unit volumenot more than twice the bulk tin concentration in moles per unit volume.

The surface tin concentration is preferably measured by XPS. It suitablyrefers to the ratio of the measured tin concentration at the surface tothe tin concentration that is nominally in the formulation. The surfacetin concentration may be not more than 1.7, is suitably not more than1.6, is preferably not more than 1.5, is more preferably not more than1.4, and, especially, is not more than 1.35.

The ITO powder may include a trace amount of sulphur. This may bemeasured by XPS. The amount of sulphur may be at least 0.1 mole %, atleast 0.5 mole % or even at least 1 mole %. The amount of sulphur at thesurface measured by XPS is preferably less than 3 mole %, morepreferably less than 2 mole %, especially less than 1.5 mole %.

The ITO of the second aspect may have any feature of the ITO describedaccording to said first aspect.

Another way of characterising the ITO described herein is in the termsof % enrichment measured as described in Example 9. Thus, in a thirdaspect, the invention provides ITO powder having a % surface tinenrichment of less than 70%, suitably less than 60%, preferably lessthan 50%, more preferably less than 40%, especially less than 35%.

The ITO of the third aspect may have any feature of the ITO of the firstand second aspects.

The invention extends to a paint, ink or resin comprising ITO asdescribed in any preceding aspect.

A particular application for ITO as produced by the method of thepresent invention or as defined in any preceding aspect is in variousoptical display devices, including electroluminescent (EL) lamps. ELlamps are thin, electrically stable multilayer devices that generallyconsist of front and rear electrodes and phosphor and dielectric layerslocated between the electrode layers. The front electrode is an actualconductive substrate that is screen or rotary screen-printed and maycomprise the ITO on a polyester film. In early EL lamps the platesconsisted of glass and ceramic, but have evolved into the thin plasticfilms that are commonly utilized today. The multi-layer structure of theEL lamp requires that the phosphor be excited with an alternatingcurrent to generate the field effect to energize the phosphor so causingit to emit light. In order to allow the light generated by the phosphorto escape, the front electrode containing the ITO must be at leastsemi-transparent. EL lamps are utilized in a wide variety ofapplications, including watches, pagers, membrane keyboards, sportsshoes, safety vests, point of sale signs, vehicles, aircraft andmilitary equipment.

Accordingly, the invention also extends to an electroluminescent lampcomprising ITO made by the method according to the invention orcomprising ITO as described in any preceding aspect.

Any feature of any aspect of any invention or embodiment describedherein may be combined with any feature of any aspect of any otherinvention or embodiment described herein mutatis mutandis.

Specific embodiments of the invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic summary of steps in the preparation of indium tinoxide (ITO);

FIG. 2 is an outline of key steps in cryogenic processing of aformulation adapted to produce ITO;

FIG. 3 is a schematic representation of a solid-solid transformation ofindium sulphate;

FIGS. 4 a to 4 c show the structure of In₂O₃, In₂O₃ doped with tin andan ITO structure with oxygen vacancies filled;

FIG. 5 illustrates the effect of added polymer on powder resistivity forthree levels of polymer additions;

FIG. 6 is a schematic representation of apparatus for measuringconductivity.

FIG. 7 is a schematic representation of a crucible; and

FIG. 8 is a schematic representation of a continuous belt furnace.

Cryogenic processing or a freeze-drying method is used to synthesise ahighly transparent and low resistivity indium tin oxide (ITO) powderthat may be used in screen printable inks. In broad terms, the processcomprises spraying a homogeneous aqueous precursor solution, comprisingsalts adapted to produce ITO in the process, by atomisation into liquidnitrogen or a cold gas whereupon the droplets formed are rapidly frozen.The droplets are conditioned by annealing and then freeze dried toremove frozen ice by sublimation leaving behind a molecular mixture ofthe precursor salts as a dry powder. This mixture is then subjected to athermal treatment to effect a transformation to the desired mixed metaloxide ITO.

The process is described in detail below, with reference to the summaryin FIG. 1.

The precursor salts selected for step (A) of the process must have highsolubility in water; be capable of being freeze dried in step (D) atconventional shelf temperatures (0-50° C.) and pressures (100-500 mbar);and be capable of being calcined in step (E) to yield ITO withoutmelting which would destroy the structure generated during freezing(step (B)) and annealing (step (C)) processes. It has been found thatindium salts such as InCl₃, In(NO₃)₃ and In₂(SO₄)₃ alone collapse ifthey are freeze dried at temperatures above around −30° C. (due toliquification of water in the structure) which makes their usepotentially time-consuming and expensive. However, it has been foundthat the addition of ammonium sulphate to the aqueous formulation instep (A) results in formation of a solution of (NH₄)In(SO₄)₂ which canbe freeze dried at a desired temperature in a reasonable time scale.Thus, in the formulation of step (A), the indium compound is provided byinclusion of ammonium sulphate together with an indium salt, inparticular indium sulphonate (NH₄)In(SO₄)₂.

Similarly, tin in higher valence states, such as SnCl₄, is prone tocollapse during freeze-drying and, additionally, the use of halides isundesirable because of potential for corrosion of the furnace duringstep (E). It has been found that the tin dopant can be successfullyintroduced using SnSO₄ as a precursor. A less preferred alternative istin (II) fluoride (SnF₂).

In a preferred embodiment, the formulation used in step (A) comprisesIn₂(SO₄)₃ and ammonium sulphate (which produce In(NH₄)(SO₄)₂) togetherwith SnSO₄.

In step (B), the formulation prepared in step (A) is sprayed byatomisation into liquid nitrogen whereupon drops are rapidly frozen toyield small intimately mixed particles of the precursor salts with icecrystals. The cryogenic processing described essentially uses phasetransformations to generate fine microstructure. The key steps in theprocess are illustrated in FIG. 2. Very high super-cooling is used asthe driver for phase separation. A major feature therefore, is thatcreation of the particle is not governed by chemistry or mixing but bytemperature and cooling rate that may be more reproducible and easier tocontrol. Ultimately complete freezing leads to the formation of a newsolid phase that is an intimate mixture (usually on a molecular level)of the starting precursors. This in turn is mixed with ice crystals on amicroscopic level leading to the formation of fine microstructure. Thismixing at different levels ultimately leads to the formation of ITO thatis different to ITO made by other known means.

As described above, some salts may collapse when freeze dried in step(D) and this is undesirable, since particles prepared may then bedifficult to re-disperse. The origin of the collapse is that duringfreezing in step (B) all or part of the ice fails to crystallise butforms a vitreous glass. As a consequence, during freeze drying therecomes a point when the frozen structure liquefies and falls apart,leading to collapse of the structure formed by the precursor salts.

To obviate the risk of collapse, step (C) is undertaken, wherein thefrozen particles prepared in step (B) are subjected to low temperatureannealing by conditioning the frozen powder at temperatures between −30to −20° C. for 30 minutes to 2 hours. The higher the temperature ofannealing, the shorter the annealing time required. It has been shown,by Isothermal DSC, that this treatment causes glassy parts of the frozenparticles to crystallise, wherein water is transformed to ice andpossibly, although not necessarily, there may be recrystallisation ofthe solute. This transition is called devitrification which occurs at atemperature in the range −35° C. to −20° C. and is accompanied by arelease of heat. Devitrification results in removal of water from theglassy parts of the frozen particles with the net result that the Tg ofthe frozen particles is raised, making sublimation of the ice in step(D) possible without collapse.

An alternative, but much less preferred embodiment, involves storing thefrozen particles of step (b) at low temperature for a long time.However, whilst crystallisation of water is thermodynamically favoured,it is kinetically very slow at low temperatures.

In step (D), the frozen crystalline water is removed without disruptionof the newly created morphology. This is achieved by sublimation of thesolvent phase in a process called lyopholisation or, from an engineeringperspective, freeze-drying. By converting the solvent phase directly tovapour without a liquid intermediate ensures minimal disruption of thesolid phase i.e. capillary pressures and secondary growth are avoided.The net result is almost perfect preservation of the solid phasemorphology.

Freeze-drying may be undertaken at a temperature in the range 0 to 50°C. and a pressure in the range 100-500 mbar. In general terms,free-drying is undertaken under conditions such that the materialtreated does not melt (which may be a possibility if the freeze dryingtemperature is too high).

In step (E), the freeze dried In(NH₄)(SO₄)₂/SnSO₄ material is subjectedto a thermal treatment which comprises calcination in a furnace at atemperature in the range 800 to 1000° C. at ambient pressure. Thethermal decomposition of the In(NH₄)(SO₄)₂ has been shown by DSC to takeplace in three steps. The first starts below 100° C. and is completed by200° C. and comprises removal of bound water. Next, at 300° C., thethermal decomposition of ammonium sulphate begins which appears to be athree-step process; the process is completed by 600° C. At 700° C. thedecomposition of indium sulphate begins and is complete by 800° C. Thedecomposition is believed to result in the production of the cubic phaseof indium oxide, together with sulphur trioxide, sulphur dioxide andoxygen. As shown in FIG. 3, the solid-solid transformation described isbelieved to proceed via a gaseous interface.

In the case of tin(II)sulphate, at 350° C., DSC reveals a phasetransition which is believed to be a change in its crystal structure.Thermal decomposition occurs sharply at 490° C. that is believed to bein accordance with the following equation:

SnSO₄(s)→SnO₂(s)+SO₂(g)

Thus, calcination temperatures of greater than 800° C. are required forcomplete decomposition. Calcination of the material can be achievedwithout melting.

After calcination, it has been observed that there are no XRD lines orpattern due to SnO₂ and that the normal XRD pattern for indium oxide hasbeen slightly displaced indicating some lattice substitution of tin forindium. The colour of the mixed oxide is pale green compared to thecanary yellow of indium oxide when not doped by tin. Thus, this confirmsthe synthesis of indium tin oxide.

The doping of the indium oxide lattice with a higher valence metal thanIn3+ (i.e. Sn4+) introduces electrons into the system by virtue of thecharge neutrality as illustrated below using the Kroger and Vinknotation:

SnO₂+In_(In) ^(x)→Sn_(In) ^(•)+In₂O₃+e′

As the valence band is filled, the electrons go into the conduction band(cb). Consequently, the ITO is an intrinsic n-type semi-conductor andmore doping should lead to higher electrical conductivity.

FIG. 4( a) illustrates the structures of In₂O₃, which is the C2 rareearth type structure; FIG. 4( b) shows substitution of two In³⁺ ions forSn⁴⁺ ions.

Due to the existence of oxygen vacancies as illustrated in FIG. 4, it isnot inevitable that more doping will increase conductivity since in thecalcination step (E) molecular oxygen produced (see FIG. 3) can fill thevacancies, as illustrated in the equation below (and in FIG. 4( c)wherein one of the oxygen vacancies has been filled with O²⁻), strippingelectrons from the conduction band in the process

O₂(g)+4e′+2V _(o) ^(x)→2O″_(o)

As a consequence, it is possible to prepare indium oxide which is highlydoped with tin but which, nonetheless, has relatively poor (compared toother materials prepared as described herein) electrical properties.

This problem may be addressed by incorporating into the formulation usedin step (A) an oxygen scavenging material which can react with excessoxygen from the sulphate decomposition according to the followingequation

SO₃→SO₂+½O₂

An organic material that could be sacrificially consumed duringcalcination was the chosen route. The selected candidate for this was awater-soluble polymer. This is easily mixed with the precursor solutionprepared in step (A) and during the cryo-processing of step (B) staysintimately mixed. Upon calcination in step (D) it combines with the SO₃or O₂ to produce SO₂, CO₂ and H₂O. If a hypothetical carbon chainpolymer is used, the following reactions may take place:

(−CH²⁻)+3SO₃→3SO₂+CO₂+H₂O

2(−CH²⁻)+3O₂→2CO₂+2H₂O

If sufficient polymer is added this should eliminate the problem andpreserve the oxygen vacancies.

Preferred polymers for use as scavengers have a relatively high Tg tomake the formulation for use in step (A) more handleable and are watersoluble and compatible with the low pH and ionic strength of thesolution used in step (A). Polyacrylic acid (PAA) and polyacrylamide(PAM)can be used.

A particularly preferred polymer is a 50 wt % polyacrylamide aqueoussolution comprising polyacrylamide of molecular weight about 10,000 amuand a Tg of 140° C. This can be added directly to the formulation foruse in step (A) in the desired amount and requires minimal dissolutiontime.

The polymer was formulated to the indium sulphate in accordance with thefollowing equation:

5In₂(SO₄)₃+2(—CH₂CHCONH₂)→5In₂O₃+15SO₂+6CO₂+5H₂O+N₂

Thus, a ratio of 518 g (1 mole) of indium sulphate to 28.4 g (0.4 molesof the repeat unit) of polyacrylamide is just sufficient to oxygenbalance the system. Note that the ammonium sulphate is not referenced inthe above equation since it decomposes before the indium sulphate and sois not relevant. Tin sulphate is not referenced since it decomposeswithout producing O₂ or SO₃.

In order to gauge the effect of polymer addition on ITO prepared,samples were prepared with 5% mole equivalents tin and polymer in therange of 50-150% mole equivalents of oxygen balance. The samples werestudied by Differential Thermal Analysis/Thermogravimetric Analysis.

It was found that with polymer present at 150% of the oxygen balance,the decomposition of the ammonium sulphate was largely unaffected;however, the indium sulphate decomposition was complete at little over600° C. Thus, the polymer had in effect lowered the calcinationtemperature by about 200° C.

It has been found that addition of polymer as described fundamentallychanges the nature of the decomposition. Samples were calcined at 900°C. as before and compared to those processed without polymer. The effectis quite dramatic (as illustrated in FIG. 5) with an almost ten-foldincrease in the performance of the powder in the resistivity test ofFIG. 6 described hereinafter. The powder resistivity is found to comparefavourably with commercially available ITO but, advantageously, this canbe achieved at lower powder volume fractions.

The level of polymer addition affects properties of the ITO. It ispossible with a large excess of polymer to sinter the ITO giving highlyconductive, low surface area dense particles. Similarly, it is possibleto use less polymer and retain the fluffy, high surface area obtainedwithout polymer addition.

The following analytical methods may be used to analyse materialsprepared as described herein.

Analytical Method 1—Measurement of Resistivitv/conductivitv and Density.

The powder electrical conductivity was measured by compacting a smallsample if material and measuring the resistance of it. The arrangementfor measuring the conductivity is shown in FIG. 6 and this provides aconvenient and rapid means of assessing the viability of a particularpowder.

The sample 50 to be tested is first weighed and inserted between twopistons 52, 54 held in place with a glass sleeve 56. A micrometer 55 isused to measure the position of a ram 60. The voltage and current aremeasured and displayed as a resistance (R) whilst an incrementallyincreasing force is applied to the sample, up to a maximum force of 5.4MPa. The simple cylindrical geometry allows the powder resistivity (r)to be measured by the relationship

$r = \frac{RA}{d}$

where A is the cross-sectional area of the sample holder and d is themeasured distance. The powder volume fraction is calculated from thefollowing

$\varphi = \frac{dA}{\rho_{o}}$

where ρ_(o) is the density of crystalline indium oxide (taken as 7.1gcm⁻³). In a typical test the resistivity as a function of powder volumefraction is plotted. This gives information not only on the electricalproperties but also how compactable the material is.

Analytical Method 2—Measurement of BET Surface Area

The BET surface area was measured by first outgassing the sample overnitrogen at 100° C. and then measuring the nitrogen sorption at liquidnitrogen temperature using a Micromeritics APSP2400.

Specific examples of the preparation of ITO are provided below.

In each of the following examples which involve calcinations in air, afrusto-conical crucible as shown in FIG. 7 was used for containingsamples. This has diameters “a” and “b” of 40 mm and 78 mm respectively;and a height “c” of 65 mm.

The bed depth of material calcined in the crucibles was 20 mm-30 mm inthe processes of Examples 1 to 6.

Unless otherwise stated all materials were used as received from AldrichUK. The indium salt described was obtained from the Indium Corporationof the USA.

EXAMPLE 1

Anhydrous indium(III) sulphate (57.5 g, 0.111 mol) was slowly added withagitation to demineralised water (240 g, 13.3 mol) at ambienttemperature. Ammonium sulphate (14.75 g, 0.111 mol) and tin(II) sulphate(2.5 g, 0.012 mol) were subsequently added. Stirring was continued untila clear slightly yellow solution was obtained.

The solution was sprayed into boiling liquid nitrogen. using a dropgenerator, which consists of a metal plate which allows predrilled holesof variable size to be inserted. In the present example an arrangementwith 5×0.5 mm holes was used. At the end of the process the excessliquid nitrogen was carefully decanted and frozen particles recovered. Anarrow frozen particle size distribution with no particles below 100 μmwas produced.

The frozen droplets were placed onto trays which were pre-cooled in abatch freeze dryer at −40° C. The frozen powder was spread evenly ontoeach tray to a bed depth of 10-15 mm.

Annealing and freeze drying were carried out in a conventional batchfreeze dryer which utilises a 24 hour cycle using a fixed programme. Theproduct was first warmed to −40° C. and held there for 30 minutes. Theshelf temperature was then raised to −25° C. and held there for 1 hourto anneal the product. The temperature was then lowered to −40° C. andheld for a further 10 minutes. This completed the annealing process.

A vacuum was then applied 0.13 mBar(100 mTorr) and the shelf temperatureraised to 25° C. over a period of 2 hours. It was then held at thistemperature for a further 4 hours. The temperature was then raised to40° C. over a period of 2 hours and then held there until the end of thedrying period (a total time of 20-24 hours). The product leaving thedryer was completely free of ice.

The dried ITO precursor was then calcined by placing material incrucibles that were then quickly placed into a muffle furnace set at900° C. in ambient air. The product was removed from the furnace after 1hour and allowed to cool to ambient temperature. It was observed thatthe initially white precursor was green after calcination and the volumeof material was reduced by a factor of about three-quarters.

The resistivity at maximum load, measured in accordance with AnalyticalMethod 1 above was 17.7 Ω.cm and the powder volume fraction was 25.8%.The BET surface area, measured in accordance with Analytical Method 2,was 12 m²/g.

The ITO produced may suitably be used as an antistatic.

EXAMPLE 2

The same formulation and procedure as described in Example 1 was usedexcept that at the end of the calcination step the powder was quenchedby discharging the hot powder from the muffle furnace directly onto ametal tray at room temperature. The resistivity and powder volume were5.3 Ωcm and 21.1% respectively.

EXAMPLE 3

The following formulation was prepared by the method described inExample 2 and analysed as described in Analytical Methods 1 and 2. Theresistivity and powder volume were 0.42 Ω.cm and 19% respectively.

Indium(III) Sulphate 57.5 g, 0.111 mol Ammonium Sulphate 14.75 g, 0.111mol Tin(II) Sulphate 2.5 g, 0.012 mol Water 240 g, 13.3 molPolyacrylamide Solution* 9.6 g, 1.5 mole equiv. *The polycarylamidesolution comprised a 50% wt solution in water of a polymer of molecularweight 10,000 amu. 1 mole equivalent is 64 g of this solution.

EXAMPLE 4

The same formulation as in Example 3 was used. Here the precursorsolution was placed in a pressurised container and sprayed via anatomiser (1-2 bar) into a Dewar vessel containing liquid nitrogen whichwas agitated. Finer frozen particles (mean size 100 μm) could beprepared. The powder was analysed in accordance with Analytical Methods1 and 2.

The resistivity and powder volume were 0.32 Ω.cm and 25% respectively.

EXAMPLE 5

The same formulation as in Example 1 was atomised into a counter-currentof cold gas in a 3 m spray tower at a temperature of −95° C. Frozenpowder was removed from the bottom of the tower continuously. This wasfurther processed as described in Example 1 and tested as in AnalyticalMethods 1 and 2.

The powder resistivity and volume fraction were 12.1 Ω.cm and 23%respectively. It will be appreciated that the method used producesparticles which compare favourably with that produced using the methodof Example 1.

EXAMPLE 6

A formulation optimisation study was carried out to obtain the bestcombination of properties for use in inks. Materials of low resistivityand high surface area were sought. It has been found that smallerparticles with high surface area do not scatter visible light as much aslarger particles. Therefore a combination of high surface area (leadingto better transparency when in an ink) and low resistivity is desirable.

The key variables identified were: tin content (or amount of doping);polymer content (essentially the amount of reduction); calcinationtemperature; and the concentration of solids in solution.

The levels of the variables studied were as follows:

Tin Content, 1%, 5% and 10% mole equivalents in ITO;Polymer Content 1.25, 1.5 and 1.75× mol. equivalents of In₂(SO₄)₃

Calcination Temperature (Tcal(° C.)) 800° C., 900° C. and 1000° C.

Concentration of Solution 20% and 33% of indium sulphate in water.

Approximately 100 g of precursor solution were prepared at each level ofthe formulation variables giving a total of 18 distinct solutions. Eachof these was processed as described in Example 2 except for thecalcination step. At this point each of the 18 dried precursor sampleswas divided into three separate lots. The first lot of 18 was calcinedat 800° C., the next at 900° C. and finally the remainder at 1000° C.All samples had their resistivity and BET measured in accordance withAnalytical Methods 1 and 2.

The data is summarised in Table 1 below.

The data was analysed and assessed. It was concluded that solutionstrength was of little effect on the measured variables. Low tin contentyielded very poor resistivity and all examples were rejected. It wasnoted that some materials prepared possess, simultaneously, high surfacearea and low resistivity.

EXAMPLE 7

The formulations of Example 6.22, 6.27, 6.28, 6.32, 6.46, 6.50, 6.51 and6.52 were selected for assessment on larger scale samples (5 times scaleup; 250 g samples) and such samples were prepared in accordance with theprocedure in Example 2 and re-tested. The bed depth of the samples(distance “d” in FIG. 7) was 50 mm. Results are provided in Table 2below.

Example 7.2 was prepared many times with similar properties and waschosen as the optimum formulation with 900° C. as the calcinationtemperature.

To optimise properties of ITO prepared it is believed to be important toconsider the bed depth in product containers. If the bed depth is highand high amounts of polymer are used and calcinations are carried out athigh temperature there is a risk of sintering the primary particles and,furthermore, the properties of ITO produced may be detrimentallyaffected. This may be explained on the basis that reducing gases areproduced from the precursor during calcinations and, consequently, theITO towards an upper end of a bed is contacted by a greater amount ofthis gas than would be the case in a shallower bed. Consequently, thereis a greater risk of over reduction, thereby affecting properties.

TABLE 1 Polymer Example Tin (mol· T_(cal) Conc. Res · BET No (%) equiv)(° C.) (%) Ω · cm (m²/g) 6.1 1 1.25 800 20 797.3 33.3 6.2 1 1.25 800 33676.5 29.3 6.3 1 1.25 900 20 6.57 9.8 6.4 1 1.25 900 33 7.7 13 6.5 11.25 1000 20 0.7 <5 6.6 1 1.25 1000 33 2 5.3 6.7 1 1.50 800 20 291.635.2 6.8 1 1.50 800 33 23.2 25 6.9 1 1.50 900 20 1.08 7.5 6.10 1 1.50900 33 1.1 7 6.11 1 1.50 1000 20 0.14 <5 6.12 1 1.50 1000 33 0.2 <5 6.131 1.75 800 20 1.48 10.1 6.14 1 1.75 800 33 1.12 7.4 6.15 1 1.75 900 200.49 <5 6.16 1 1.75 900 33 0.36 <5 6.17 1 1.75 1000 20 0.18 <5 6.18 11.75 1000 33 0.08 <5 6.19 5 1.25 800 20 33.7 37.9 6.20 5 1.25 800 3312.1 38.4 6.21 5 1.25 900 20 1.49 25.6 6.22 5 1.25 900 33 0.7 22.1 6.235 1.25 1000 20 0.21 6.5 6.24 5 1.25 1000 33 0.35 9.9 6.25 5 1.50 800 2011.5 42.2 6.26 5 1.50 800 33 3.24 33.1 6.27 5 1.50 900 20 0.42 16.8 6.285 1.50 900 33 0.34 16.3 6.29 5 1.50 1000 20 0.11 <5 6.30 5 1.50 1000 330.2 <5 6.31 5 1.75 800 20 0.94 25.5 6.32 5 1.75 800 33 0.77 33.3 6.33 51.75 900 20 0.24 6.7 6.34 5 1.75 900 33 0.15 <5 6.35 5 1.75 1000 20 0.07<5 6.36 5 1.75 1000 33 0.09 <5 6.37 10 1.25 800 20 9.68 39.1 6.38 101.25 800 33 10.57 42.9 6.39 10 1.25 900 20 1.48 25 6.40 10 1.25 900 331.86 24.8 6.41 10 1.25 1000 20 0.44 <5 6.42 10 1.25 1000 33 0.36 10 6.4310 1.50 800 20 3.33 38 6.44 10 1.50 800 33 0.98 34 6.45 10 1.50 900 200.21 8.4 6.46 10 1.50 900 33 0.23 12.1 6.47 10 1.50 1000 20 0.18 <5 6.4810 1.50 1000 33 0.22 10.7 6.49 10 1.75 800 20 0.97 32.1 6.50 10 1.75 80033 0.38 29.3 6.51 10 1.75 900 20 0.25 10.7 6.52 10 1.75 900 33 0.25 12.16.53 10 1.75 1000 20 0.21 27.1 6.54 10 1.75 1000 33 0.17 <5

TABLE 2 Polymer Example Similar Tin (mol · T_(cal) Conc Res · BET NoExample (%) equiv) (° C.) (%) Ω · cm (m²/g) 7.1 6.22 5 1.25 900 33 0.722.1 7.2 6.27 5 1.50 900 20 0.13 28 7.3 6.28 5 1.50 900 33 0.23 12.1 7.46.32 5 1.75 800 33 0.51 18.2 7.5 6.46 10 1.50 900 33 0.81 21.6 7.6 6.5010 1.75 800 33 0.27 18.7 7.7 6.51 10 1.75 900 20 0.16 5 7.8 6.52 10 1.75900 33 0.25 5

EXAMPLE 8

The following formulation was prepared by using the method of Example 1as far as the freeze drying stage. Calcination was carried out asfollows: alumina trays were used to hold the precursor for thecalcination step. 50 g of precursor was placed in each tray 100 (FIG. 8)and placed on the belt 102 of a tunnel furnace 104 which consisted ofthree zones, as shown in FIG. 8. In the first zone 106 there was noheating; the product passes into this zone via a nitrogen curtain 108.The product then enters the hot zone 110 which is a metal mufflecontrolled at 900° C. and a cover gas of nitrogen 112 is used. Exhaustgases are removed via a venturi 114. After the heating zone the producttray passes into a cooling zone 116 using circulating water to removeheat from the product. Nitrogen 118 is again used as a cover gas.Typically, distance x is 3.4 m, distance y is 1.4 m and the belt speedis 5 cm/minute.

The product ITO emerges from the cooling zone 116 below 50° C. and canbe transferred directly to containers.

The powder properties were assessed in accordance with AnalyticalMethods 1 and 2.

Indium(III) Sulphate 575 g (1.11 mol) Ammonium Sulphate 147.5 g (1.11mol) Tin(II) Sulphate 25 g (0.12 mol) Polacrylamide Solution 86 g (1.34mol. equiv) Water 2400 g (133.3 mol)

The powder resistivity was 0.25 Ω.cm at a powder volume of 25%. The BETsurface area was 15 m²/g.

EXAMPLE 9

Samples of ITO were prepared in accordance with the method of Example 8using tin concentrations of 1 mol % (Example 9a), 5 mol % (Example 9b)and 10 mol % (Example 9c) of the indium concentrations.

These samples and two commercially available ITO samples (Examples CA1and CA2) were analysed by X-ray photoelectron spectroscopy (XPS) toassess the surface tin and surface sulphur concentrations. Results areprovided in Table 3 below.

TABLE 3 Sn/In Nominal Sn/In XPS Example No (mole %) (mole %) %Enrichment Smole % 9a 1.0 1.37 37% 1.4 9b 5.0 6.74 35% 1.2 9c 10.0 12.9129% 1.1 CA1 1.0 2.0 100%  0 CA2 10.0 17.3 73% 0

Table 3 details the mole % surface enrichment of the ITO that iscalculated according to the following equation:

% enrichment=[100(Sn/InXPS(mole %)+(Sn/In Nominal(mole %))]-100%(Measured mole %-Nominal mole %)×100/(Nominal Mole %)

Sn/InXPS refers to the surface tin to indium ratio measured by XPS.

It will be appreciated that the lower the % enrichment, the moredesirable the ITO may be.

It will be noted also that the presence of sulphur may also be used todistinguish the ITO produced by the method described from that producedby other methods.

EXAMPLE 10

Samples (A series) of EL lamps were prepared using the ITO prepared asdescribed in Example 8, while a second, comparative set (B series) wereprepared using the ITO L-1469-2, commercially available from Mitsubishi.The results of testing on these samples are shown in Table 4.

TABLE 4 Light Output Amperage Draw Sample (Cd/m2) (Milliamps)Colour-Co(x-y)¹ LAR² A1 90.5 3.21 0.202-0.475 28.19315 A2 95.2 3.310.202-0.475 28.76133 A3 94.7 3.30 0.202-0.476 28.69697 B1 28.2 1.280.193-0.433 22.03125 B2 34.1 1.30 0.193-0.432 26.23077 B3 31.2 1.260.192-0.432 24.76190 ¹Measured using the 1931 CIE standard. ²LAR islight output divided by amperage draw.

As shown in Table 4, the EL lamps containing the indium tin oxide of thepresent invention provide superior light output and other propertiesthan the EL lamps containing the commercially available indium tinoxide.

1. A process for the preparation of indium tin oxide (ITO) whichincludes the steps of: (a) causing a liquid formulation which includes asolvent to form a solid, wherein the formulation includes: (i) an indiumcompound, a tin compound and ammonium sulphate; or (ii) (NH₄)In(SO₄)₂and a tin compound; (b) conditioning the solid; (c) causing removal ofsolvent from the solid prepared in (b); (d) calcining the solid afterstep (c) thereby to produce ITO.
 2. A process according to claim 1,wherein said liquid formulation of step (a) is an aqueous formulation.3. A process according to claim 1, wherein the ratio of the number ofmoles of indium ions in said indium compound to the number of moles oftin ions in said tin compound in said liquid formulation is in the range5 to 50; and the ratio of number of moles of ammonium ions in saidammonium compound to the number of moles of tin ions in said tincompound in said formulation is in the range 5 to
 50. 4. A processaccording to claim 1, which comprises the materials of (a)(i), whereinthe ratio of the number of moles of indium ions in said indium compoundto the number of moles of ammonium ions in said ammonium sulphate is inthe range 0.6 to 1.5; and the ratio of the number of moles of tin ionsin said tin compound to the number of moles of ammonium ions in saidammonium sulphate is in the range 0.01 to 0.1.
 5. A process according toclaim 1, wherein said liquid formulation of step (a)(i) includes atleast 0.4 wt % of said tin compound, at least 2 wt % of ammoniumsulphate, at least 10 wt % of said indium compound and at least 60 wt %of solvent.
 6. A process according to claim 1, wherein said tin compoundused in steps (a)(i) or (a)(ii) is a Sn(II) compound.
 7. A processaccording to claim 6, wherein said tin compound is tin (II) sulphate. 8.A process according to claim 1, which is carried out in the presence ofoxygen scavenging means which is arranged to scavenge and/or react withoxygen produced in the process to reduce the amount of oxygen that maybe incorporated into the ITO prepared.
 9. A process according to claim8, wherein said scavenging means is a chemical compound which is able toreact with decomposition products produced in step (b) of the process.10. A process according to claim 8, wherein said scavenging means is awater-soluble polymeric material.
 11. A process according to claim 10,wherein the amount of scavenging means in said formulation is 0.9 to 2times the amount required to fully reduce the products of thedecomposition of indium sulphate to indium oxide.
 12. A processaccording to claim 1, wherein step (a) includes causing the liquidformation to cool to a temperature which is at or below the freezingpoint of the liquid formulation.
 13. A process according to claim 1,wherein in step (a) the liquid formulation is introduced into a lowtemperature environment which is at a temperature of less than −25° C.14. A process according to claim 1, wherein in step (a) the liquidformulation is caused to form particles of solid.
 15. A processaccording to claim 14, wherein less than 10 wt % of the particles formedin step (a) and treated in step (b) have a particle size of less than100 μm.
 16. A process according to claim 14, wherein a major amount ofthe particles are in the range 100 μm to 2 mm.
 17. A process accordingto claim 1, wherein in said conditioning step (b) a part of the solid iscaused to undergo a physical change.
 18. A process according to claim 1,wherein in said conditioning step (b) the crystallinity of the solventis increased.
 19. A process according to claim 1, wherein in step (b),the temperature of the solid is raised by at least 5° C.
 20. A processaccording to claim 1, wherein step (c) includes causing sublimation ofthe solvent.
 21. A process according to claim 1, wherein step (d)includes subjecting the solid to an environment wherein the temperatureis at least 400° C. and is less than 1200° C.
 22. A process according toclaim 1, wherein the ITO produced in the process has a powder resitivityin the range 0.1 to 0.5 Ω.cm measured at less than 30% volume fraction;and a BET surface area of less than 35 m2/g.
 23. Indium tin oxide (ITO)powder having a surface tin concentration in moles per unit volume notmore than twice the bulk tin concentration in moles per unit volume. 24.Indium tin oxide (ITO) powder having a % surface tin enrichment of lessthan 70%.
 25. ITO according to claim 23, which includes a trace ofsulphur.
 26. ITO according to claim 23, which includes at the surface0.1 to 3 mole % of sulphur.
 27. ITO according to claim 23, having apowder restitivity in the range 0.1 to 0.5 Ω.cm measured at less than30% volume fraction.
 28. ITO according to claim 23, wherein the BETsurface area is less than 35 m2/g.
 29. A paint, ink or resinincorporating ITO made in a process according to claim
 1. 30. Anelectroluminescent lamp comprising ITO made by the process according toclaim
 1. 31. An electroluminescent lamp comprising ITO as claimed inclaim
 23. 32. A process, indium tin oxide, a paint, ink or resin and anelectroluminescent lamp, each being independently substantially ashereinbefore described with reference to the examples.